Somehow, I'm overweight, but also very obese? Please help

Hi. I was recently made aware by my doctors that I am both extremely obese and unhealthily underweight.

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I am a little under 4'11", 18yrs old, biologically female, 87lbs, with a 35 inch waist and a 36 inch hip. I walk between 15,000 to 22,000 steps per day (don't know how much it is in miles or kilometers though), and do mild weight training once per week. I consume in the range of 1600-1700 cals. My BMI is 17.6 and my waist-hip ratio is .97.

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I was told that my waist-hip ratio means that I am extremely obese, but my BMI says that I am very underweight. I was advised to gain weight by one doctor, and lose it by another. I am extremely confused as to how this is possible. Can anyone please help explain how this is possible?